The invention relates to an apparatus and method for stabilizing the pointing direction and position of an electromagnetic (EM) beam, namely a laser beam, at a unique, predetermined point in space, and more particularly, to an apparatus and method for controlling both the short and long term stability of the output of a laser-pumped laser by actively stabilizing the pump beam position and pointing direction within the active gain volume in the laser being pumped.
Many applications of laser beams require the beam to be directed to a specific point in space (a target) and to be accurately maintained in that position for a time needed to perform a specific function. The direction of the beam propagation path at that point in space may also need to be controlled. For example, in order to ensure the stable operation of laser-pumped lasers, it is often necessary to ensure a precise degree of overlap of the pump beam volume with the active gain volume of the laser being pumped for significant periods of time. This is especially true, for example, when the optical Kerr effect plays a role in mode-locking the laser-pumped laser, since even slight variations in the degree of overlap of the pump beam volume with the active gain volume can result in deleterious effects on laser operation.
Changes in the pump beam position in the gain medium of the laser-pumped laser arise from a number of factors. The changes can be induced by mechanical vibration and/or thermally induced expansion or distortion within the laser cavity of the pump laser itself. They can also result from mechanical vibration and/or thermally induced expansion or distortion of the optical components that the pump beam engages as it propagates to the active gain volume within the laser being pumped. Additional deleterious effects result from dimensional changes caused by mechanical vibrations and/or thermally induced expansion or distortion of the surface to which both lasers are attached, causing the pump laser beam to shift in position relative to the laser being pumped. Further operational defects can be caused by changes within the laser-pumped laser cavity which effect a positional shift of the active gain volume within the gain medium itself. In like manner, mechanical vibrations and/or thermally induced expansion or distortion can produce deleterious effects at the location where the laser beam is directed to perform a predetermined function.
Active and passive commercial systems are known for stabilizing the beam pointing direction of a laser beam with respect to the cavity of the laser pump source. Active systems generally comprise a single beamsplitter and single beam position sensor such as a quadrant detector, mounted or attached to the resonator of the pump laser. The beamsplitter and quadrant detector are arranged so that the beamsplitter directs a portion of the output beam onto the quadrant detector, the beam being fixed at a point within the cavity. The detector, along with associated hardware and electronics, provide a feedback signal that is used to make corrections to two orthogonal angular adjustments on a mirror in the resonator cavity of the pump laser for correcting its beam pointing direction. While this type of stabilization technique can be effective in stabilizing the beam pointing direction of the laser used to pump another laser, it addresses only part of the problem of drift in laser-pumped lasers because the point of reference where the quadrant detector is located is outside the cavity of the pump laser itself. Moreover, it fails to deal with the other sources of instabilities in laser pumped-lasers, mentioned above, that can shift the position of the pump beam within the active gain volume of the laser being pumped. Although a single beamsplitter, a quadrant detector and an actuated mirror can control the beam pointing direction of a laser beam, and thereby fix its position at a specific location, this approach is not capable of fixing the beam pointing direction at that location as well, which is crucial when beam drift is due to changes in pointing direction.
Included among the passive techniques used to stabilize the pointing direction and position of a laser beam at a specific point in space are the use of large, interlocking optical tables which provide a single surface for the laser source, processing optics and target; the use of vibration isolation legs to minimize the coupling of mechanical vibrations between the floor and the equipment in contact with the surface of the table; and the use of low expansion materials like Superinvar.TM. to reduce the sensitivity of components to thermal changes in the environment. Unfortunately, these types of systems are often very expensive, and have limited effectiveness.
Accordingly, the inventor has recognized a need for methods and equipment which accomplish the beam control and stabilization described above without the disadvantages of the existing systems that have been set forth.